1. Field of the Invention
This invention relates to flexible, open-celled, free-rise polyurethane ("PU") foams made by molding or by continuous casting methods. When formulated in accordance with the present invention, the PU foams possess enhanced flame retardant properties. Such ignition resistant foams are particularly suitable for use in furniture, bedding, and carpet padding.
2. Description of Related Art
Flexible PU foams, of both the conventional and high resilient types, are produced from formulations typically containing an isocyanate reactant, one or more blowing agents including water, a foam stabilizer, catalysts, and polyether polyols. High resilient ("HR") foams are those which exhibit a rapid recovery from extreme compression. As compared with conventional foams, HR foams exhibit a higher resilience (55% to 65% for HR, compared with 40% to 55% for conventional) and a higher modulus (2.2 to 2.7 for HR, compared with 1.8 to 2.3 for conventional), where the modulus is defined as the ratio of the 65% Indentation Force Deflection ("IFD") measurement to the 25% IFD measurement.
PU foams are formed by the effervescent action of a gas on the product of a polymerization reaction. Specifically, isocyanate groups react with water to generate carbon dioxide gas. This gas is dispersed and, to a large extent, retained during the subsequent polymerization--which involves the reaction of additional isocyanate with hydroxyl groups of the polyols. At the time the polymerization is essentially complete, the foam gels (i.e., becomes solid) and the cells are usually broken open by the heat of reaction and the pressure of trapped gases to produce the well-known open cell structure of flexible PU foams.
Flexible PU foams can readily be ignited by an open flame ignition source. It is therefore desirable to identify materials which, when added to the foam formulation, will increase ignition resistance without deleteriously affecting other critical properties of the foam.
The use of melamine as a fire retardant additive for various polymers has been known for many years. In PU foams, the addition of melamine in intermediate to large amounts, i.e., approximately 20 to 200 phr (parts by weight per hundred parts of polyol), has been demonstrated to be useful in enhancing the ignition resistance of both flexible and rigid foams.
In contrast to these so-called "melamine foams," other PU foam formulations traditionally have achieved satisfactory flame resistance ratings by incorporating standard halogenated phosphorus ester flame retardant additives, without addition of melamine.
U.S. Pat. No. 4,757,093 discloses the substitution of small amounts (approximately 1 to 3 phr) of melamine for a portion of the halogenated phosphorus ester flame retardant in flexible foam formulations, with no diminution in the degree of ignition resistance. The patent teaches, however, that no benefit accrues from the addition of melamine in amounts greater than 5 phr, since the the marginal improvement in the foam's flame resistance would not justify the additional increase in cost.
The preferred halogenated phosphorus ester additives of U.S. Pat. No. 4,757,093 include tetrakis(2-chloroethyl) ethylene diphosphate, commercially available from Olin Chemical Corporation under the tradename Thermolin TM-101. The preferred esters are disclosed to be essentially interchangeable with a blend of about 80 to 85% pentabromodiphenyl oxide, a non-ester, in an aromatic phosphate plasticizer, marketed by Great Lakes Chemical Corporation under the tradename DE-60F. The patent presents burn test data indicating that the combination of either DE-60F or a preferred ester with melamine yields equivalent flame retardant performance.
In a paper presented at the 12th International Conference on Fire Safety in 1987, BASF Corporation disclosed the use in HR flexible foam formulations of a combination of an intermediate amount (approximately 10 to 25 phr) of melamine with standard flame retardant additives in amounts of approximately 3 to 12 phr. The resulting HR foams were able to pass the California Technical Bulletin No. 117 test, adopted in 1975, for resistance to a small open flame source. BASF also disclosed the use of low levels of melamine to improve smoldering resistance of HR foams after flexing.
The use of the melamine/standard flame retardant combination, as disclosed by BASF, appears to have been restricted to HR foam technology. Such a restriction is understandable, because one effect of the additive combination is to depress the already low melting point of HR foams, to produce a melt which is difficult to ignite. The use of such additives in conventional flexible foams, which typically have higher melting points, would have been expected to be unavailing.
In addition to the fact that the us of the additive combination has been limited to HR foam technology, the resulting foams also have been demonstrated to meet only the least rigorous burn standards, e.g., California 117. The need continues for foams capable of passing the more demanding flammability tests, such as Underwriters Laboratories UL-94A, adopted in January 1980, and, especially, British Standard 5852 Pt.2 Source 5, adopted in March 1982.